Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed.
A conventional ultrasound imaging system is shown in FIG. 1. In this system, an ultrasound transducer array 2 is activated to transmit a series of multi-cycle tone bursts which are focused at the same transmit focal position with the same transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF). A series of transmit firings focused at the same transmit focal position and having the same transmit characteristics is referred to as a "packet". Each transmit beam propagates through the object being scanned and is reflected by ultrasound scatterers in the object.
After each transmit firing, the echo signals detected by the transducer array elements are fed to respective receive channels of a beamformer 4. The receive beamformer tracks echoes under the direction of a master controller (not shown in FIG. 1). The receive beamformer imparts the proper receive focus time delays to the received echo signal and sums them to provide an echo signal which accurately indicates the total ultrasonic energy reflected from a succession of ranges corresponding to a particular transmit focal position. The beamformer also transforms the RF signal into its I/Q components by means of Hilbert bandpass filtering. The I/Q components are then summed in a receive summer (not shown in FIG. 1) for each transmit firing. Hilbert bandpass filtering can alternatively be performed after beam summation.
The output of the beamformer 4 is shifted in frequency by a demodulator 6. One way of achieving this is to multiply the input signal by a complex sinusoidal e.sup.i2.pi..function.dt, where .function..sub.d is the frequency shift required. The downshifted I/Q components are then sent to a B-mode processor, which incorporates an envelope detector 20 for forming the envelope of the beamsummed receive signal by computing the quantity (I.sup.2 +Q.sup.2).sup.1/2. The envelope of the signal undergoes some additional B-mode processing, such as logarithmic compression (block 22 in FIG. 1), to form display data which is output to the scan converter 14.
In general, the display data is converted by the scan converter 14 into X-Y format for video display. The scan-converted frames are passed to a video processor 16, which maps the video data to a gray-scale mapping for video display. The gray-scale image frames are then sent to the video monitor 18 for display.
The images displayed by the video monitor 18 are produced from an image frame of data in which each datum indicates the intensity or brightness of a respective pixel in the display. An image frame may, e.g., comprise a 256.times.256 data array in which each intensity datum is an 8-bit binary number that indicates pixel brightness. The brightness of each pixel on the display monitor 18 is continuously refreshed by reading the value of its corresponding element in the data array in a well-known manner. Each pixel has an intensity value which is a function of the backscatter cross section of a respective sample volume in response to interrogating ultrasonic pulses and the gray map employed.
In the color flow mode, the I/Q components are stored in a corner turner memory 8, whose purpose is to buffer data from possibly interleaved firings and output the data as vectors of points across firings at a given range cell. Data is received in "fast time", or sequentially down range (along a vector) for each firing. The output of the corner turner memory is reordered into "slow time", or sequentially by firing for each range cell. The resultant "slow time" I/Q signal samples are passed through respective wall filters 10.
Given the angle .theta. between the insonifying beam and the flow axis, the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU v=cf.sub.d /(2f.sub.0 cos .theta.) (1)
where c is the speed of sound in blood, .function..sub.0 is the transmit frequency and .function..sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound signal.
The wall-filtered outputs are fed into a parameter estimator 12, which converts the range cell information into the intermediate autocorrelation parameters N, D, and R(0). N and D are the numerator and denominator for the autocorrelation equation, as shown below: ##EQU1## where I.sub.i and Q.sub.i are the input data for firing i, and M is the number of firings in the packet. R(0) is approximated as a finite sum over the number of firings in a packet, as follows: ##EQU2## R(0) indicates the power in the returned ultrasound echoes.
A processor in parameter estimator 12 converts N and D into a magnitude and phase for each range cell. The equations used are as follows: ##EQU3##
The parameter estimator processes the magnitude and phase values into estimates of power, velocity and turbulence. The phase is used to calculate the mean Doppler frequency, which is proportional to the velocity as shown below; R(0) and .vertline.R(T).vertline. (magnitude) are used to estimate the turbulence. The mean Doppler frequency in hertz is obtained from the phase of N and D and the pulse repetition time T: ##EQU4## The mean velocity is calculated using the Doppler shift equation below. Since .theta., the angle between the flow direction and the sampling direction, is not known, cos .theta. is assumed to be 1.0. ##EQU5## The parameter estimator 12 does not calculate the mean Doppler frequency as an intermediate output, but calculates v directly from the phase output of the processor using a lookup table. Typically the power estimates are compressed before scan conversion, e.g., using logarithmic compression (block 13 in FIG. 1)
The power or velocity estimates are sent to scan converter 14, which converts the color image frame data into X-Y format for video display. The scan-converted frames are passed to a video processor 16, which maps the video data to a display color map. The color flow image frames are then sent to the video monitor 18 for display.
In medical ultrasound systems it is necessary to apply envelope detection and/or phase detection to inphase (I) and quadrature (Q) signals, and to employ logarithmic compression or other similar types of compression for B/M-mode and color flow imaging. Because detection and compression are highly nonlinear, the associated operations require excessive computation, which makes real-time computation difficult. Therefore, to avoid performing excessive computation, it has been common in medical ultrasound to employ lookup tables in order to complete in real time the operations associated with envelope detection, phase detection and logarithmic compression. There is a need for alternative approaches to envelope detection, phase detection and compression, which reduce the amount of computation required.